The processing of logs of a selected species of tree into finished lumber requires a number of steps from the initial rough sawing of the logs to the sizing and drying of the finished lumber into uniform commercial sizes. Given the number of steps and the size of the material being processed requires that the entire process be coordinated efficiently, and that each step be completed in as short a time as possible to minimize the area at a mill that is dedicated to storage of the lumber between each of those steps, as well as prior to shipping of the finished product. Thus, ways have been sought over the years to automate and minimize the time necessary to complete each step in the production of lumber.
One of the steps that, historically, can be very time consuming and storage space extensive is the final drying process of the lumber. Depending on the thickness and species of the wood, days to months are required in the drying step. Kiln drying however has not been without problems. One of the most persistent has been, and continues to be, the ability to determine when lumber in the kiln is dry. Without a reliable process and equipment to accurately detect when the lumber is dry, lumber has traditionally been kept in the kiln longer than actually necessary. Those longer drying times in the kiln have come at a price, namely losses from overdrying and having to store a large quantity of sawn finished lumber waiting for its turn in the kiln for drying, or a more expensive solution, using more kilns. Thus, there has been ongoing research for many years for techniques and devices that more accurately determine when lumber in a kiln is dry so that it can be removed from the kiln after the shortest period of time.
In-kiln monitoring devices have been developed to provide information on two key major points during the dying process, the reduction of the core moisture content (MC) below the fiber saturation point (FSP) and the endpoint moisture content of the lumber in the kiln. The most widely used technologies (capacitance and conductance), have major drawbacks in their precision and repeatability. The capacitance method is the easiest to use, since it involves the insertion of metal electrodes through slot openings in the stack of lumber and the monitoring of changes in the capacitance to ground during the kiln drying process. Since capacitance of wood depends on grain orientation, density, wood extractives, and wood temperature, as well as to moisture content, it has inherent errors that cannot be corrected. Additionally, it is well known that wood in the drying process develops moisture gradients that distort the intended determination. The conductance method is more direct than the capacitance method in that pins are imbedded in the lumber at a number of locations and in a manner to determine core and shell moisture content. The conductance technique is limited to obtaining information from the edge of the lumber stack and is very local for each measurement point (about 25 mm between each pin). In contrast, the capacitance technique can provide "average" moisture content over the width of the stack. However, extreme moisture content values control stress development in lumber, not average moisture content values as measured by the capacitance method. Unfortunately, the resistance of wood is affected primarily by temperature, as well as some of the same variables mentioned for the capacitance. Additionally, neither the capacitance or the conductance method is accurate in measuring the fiber saturation point (FSP) in the core of pieces of lumber in the stack.
Another method that has been used for in-kiln sensing is the monitoring of the temperature drop across the load (DTL), which gives an approximation of the drying rate. This approximation is useful for an overall index of drying of softwoods (because of the high drying rate and therefore a large DTL), but cannot be used reliably for endpoint determination.
Also, many techniques have been proposed using passive acoustic emission (AE) from the lumber while drying and active ultrasonic transmission through the lumber while in the kiln. The prior art techniques for monitoring acoustic emission from the lumber have focused on the use of AE transducers to monitor development of stress in the lumber being dried which generate the acoustic emission from the lumber with AE transducers being attached directly to a board face or edge of individual pieces of lumber in the stack during drying. These techniques have demonstrated a number of distinct disadvantages.
One of those disadvantages is that those techniques each affords only very limited acquisition of AE data, on the order of only several centimeters from the transducer shoe in an individual piece of lumber in the stack. Using the prior art AE techniques to acquire sufficient data, thus requires the use of a very large array of transducers, which, if used, would be prohibitive in cost, cause time delays by the extensive set-up time required, along with installation problems with the individual transducers, the cabling requirements and the instrumentation.
A second major disadvantage is the difficulty to achieve and maintain consistent coupling between all of the transducers during set-up as well as throughout the drying process. The typical coupling material between each transducer and the piece of lumber used in prior art AE sensing is a grease-type material which is objectionable for commercial application. For rough-sawn wood, the grease over-penetrates the wood cells, and tends to lose contact pressure with the board as temperature in the kiln increases and during the drying period. Attaching the transducer with a typical hold-down damp also has proved not to be practical. Several researchers have also tried using other clamping devices which have been adequate for laboratory studies, but impractical in a commercial system because of the time, costs or special, non-standard, stacking requirements of the lumber being dried.
The only other feasible technique of attaching transducers in the prior art has been the use of an adhesive or through dry coupling. The adhesively-bonded approach has similar drawbacks to the grease couplant, such as temperature sensitivity, but also presents a major difficulty in bonding to wet material prior to the beginning of the drying process. The application and setting time of the adhesive also makes that approach wholly impractical in either the commercial or laboratory setting. Dry coupling has also proven to have two major drawbacks: loss of sensitivity of the transducer, and difficulty in applying sufficient pressure between the transducer and the lumber at all stages of the drying process to maintain suffident coupling between the transducer and the lumber. In the prior art, typical dry coupling uses elastomeric materials that can perform in a similar manner as a grease couplant in squeezing out air gaps between the transducer shoe and contacting material.
To overcome these many problems, from collecting a sufficient amount of AE data to clean and consistent coupling to the lumber during the entire drying process, requires a new approach to this on-going, persistent problem. As will be seen in the following discussion, the method and special sticker design of the present invention overcomes those problems.